You can't give unRs a type. What I usually do in this situation is to make the Rs type a Resource too. instance Resource Rs where resourceName (Rs a) = resourceName a resourceAdvance (Rs a) = Rs (resourceAdvance a) resourceStarved (Rs a) = resourceStarved a resourceSpend (Rs a) i = Rs (resourceSpend a i) resourceEarn (Rs a) i = Rs (resourceEarn a i) And then you export Rs (with some mkRs=Rs if you export it abstractly) and the Resource class. Then you can use Rs to make your heterogeneous lists and class ops to manipulate them. -- Lennart Matthias Fischmann wrote:
hi,
this is one of those situations that always make scheme and perl hackers laugh at me: i have written a piece of code that is intuitively clear, and now i am trying to turn it into something that compiles. and here it goes.
i have a type class that looks something like this:
class Resource a where resourceName :: a -> String resourceAdvance :: a -> a resourceStarved :: a -> Bool resourceSpend :: a -> Int -> a resourceEarn :: a -> Int -> a
resource types are rice, crude oil, pizza, software code, and so on. they all have a different internal structure and the same abstract interface, that's why i have defined this type class.
now i want to create a list of a type similar to
[r1, r2, r3] :: (Resource a) => [a]
but with r1 being pizza, r2 being crude oil, and so on. my first idea was this:
data Rs = forall a . (Resource a) => Rs a unRs (Rs a) = a rsName :: Rs -> String rsName = resourceName . unRs ...
and then export Rs as an abstract data type. this would allow for lists of type [Rs], which is exactly what i want.
but what is the type of unRs? or better: can i make it type at all? and isn't this solution a little redundant and verbose? should i do it like in the example for existentially quantified types in the ghc manual?
http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html
but wouldnt't the code become really messy? or should i do the type class and instances, and then do Rs the existentially quantified way, with all class methods arguments to the Rs constructor? or is there a completely different way to do this (besides using scheme or perl :-)?
thanks, matthias
------------------------------------------------------------------------
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe